Power converters are used to interface between power sources and electrical devices. Because alternating current is preferred for efficient power transmission, power sources typically provide power at alternating current. On the other hand the operation of many electrical devices requires direct current. In order to provide direct current for such devices from alternating current, the power sources, power converters need to convert the current supplied from alternating to direct, and use rectifiers for this purpose.
Bridge rectifiers are commonly used in power converters to provide full-wave rectification of alternating current. A typical bridge rectifier is the Graetz circuit or diode bridge which consists of four diodes arranged in a bridge circuit as shown in FIG. 1a. It will be noted, however, that diodes are inherently inefficient conductors and this produces a number of well known problems.
One problem resulting from the inefficiency of diodes is that they produce a forward voltage drop. This is most noticeable in low-voltage power converters where the voltage drop may be a significant proportion of the desired voltage output. Schottky diodes, which exhibit very low voltage drops, are often used to minimize the problem.
A further problem results from the heat dissipated by diodes, particularly in high frequency switching power supplies. Apart from reducing overall efficiency of the power converter, resulting high temperatures also reduce the reliability of components. Much design effort is typically required to overcome this problem, and other factors such as the dimensions of the system may be compromised as a result.
MOSFETs (Metal Oxide Semiconductor Field-Effect Transistors) are electronic switches. A gate signal may be used to switch the MOSFET between an OFF state and an ON state. In the OFF state, current is blocked whereas in the ON state, a current path is provided between the source and the drain terminals, having the characteristics of an ohmic resistor.
A prior art Power MOSFET 130 is shown in FIG. 1b. The Power MOSFET 130 consists of an electronic switch 131 having a source terminal 134, a drain terminal 136 and a gate terminal 138, and a diode 132 connected between the source 134 and drain 136 terminals in parallel to the electronic switch 131. When the MOSFET 130 is in the OFF state, current may pass only through the diode 132, however when the MOSFET 130 is in the ON state, an alternative low resistance current path is provided between the source 134 and the drain 136 terminals through the electronic switch 131. Because the closed electronic switch 131 of the MOSFET 130 behaves as an ohmic resistor, it produces less heat loss than the diode 132.
Additionally, it will be noted that MOSFETs have extremely fast switching speeds. This allows them to be used to replace diodes in a rectifier. Typically, the gate signals of the MOSFETs are synchronized with the frequency of the input voltage such that they are switched into their ON state whenever there is a voltage drop between the source 134 and the drain 138 terminals. Such rectifiers are known as synchronous rectifiers and may be used to improve the efficiency of power converters.
In bridge rectifiers, up to two of the diodes of the Graetz circuit may be replaced by MOSFETs, and their gate signal drawn directly from the input voltage. However, if the remaining two diodes are replaced by MOSFETs and also draw gate signals directly from the input voltage, it is difficult to avoid short circuiting between the input and/or the output terminals of the bridge rectifier.
There is a need therefore for a synchronous rectifier characterized by a low associated heat loss and the present invention addresses this need.